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| <h2 title="Annotation Type DoFn.RequiresStableInput" class="title">Annotation Type DoFn.RequiresStableInput</h2> |
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| <pre>@Documented |
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| public static @interface <span class="memberNameLabel">DoFn.RequiresStableInput</span></pre> |
| <div class="block"><b><i>Experimental - no backwards compatibility guarantees. The exact name or usage of this |
| feature may change.</i></b> |
| |
| <p>Annotation that may be added to a <a href="../../../../../org/apache/beam/sdk/transforms/DoFn.ProcessElement.html" title="annotation in org.apache.beam.sdk.transforms"><code>DoFn.ProcessElement</code></a>, <a href="../../../../../org/apache/beam/sdk/transforms/DoFn.OnTimer.html" title="annotation in org.apache.beam.sdk.transforms"><code>DoFn.OnTimer</code></a>, or <a href="../../../../../org/apache/beam/sdk/transforms/DoFn.OnWindowExpiration.html" title="annotation in org.apache.beam.sdk.transforms"><code>DoFn.OnWindowExpiration</code></a> method to indicate that the runner must ensure that the observable contents |
| of the input <a href="../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a> or mutable state must be stable upon retries. |
| |
| <p>This is important for sinks, which must ensure exactly-once semantics when writing to a |
| storage medium outside of your pipeline. A general pattern for a basic sink is to write a |
| <a href="../../../../../org/apache/beam/sdk/transforms/DoFn.html" title="class in org.apache.beam.sdk.transforms"><code>DoFn</code></a> that can perform an idempotent write, and annotate that it requires stable input. |
| Combined, these allow the write to be freely retried until success. |
| |
| <p>An example of an unstable input would be anything computed using nondeterministic logic. In |
| Beam, any user-defined function is permitted to be nondeterministic, and any <a href="../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a> is permitted to be recomputed in any manner.</div> |
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